Gettering is a process of removing device-degrading impurities from the active circuit regions of a semiconductor wafer. As such, gettering helps enhance the yield of VLSI manufacturing.
Generally, there are three gettering mechanisms by which impurities are removed from a device region of a semiconductor substrate. One mechanism involves precipitating the impurities. Another mechanism involves diffusing impurities through the silicon, and yet another involves trapping impurities (e.g., metal atoms) in defects such as dislocations or precipitates in an area away from the device region.
The gettering mechanisms may be placed in two general classifications: extrinsic and intrinsic. Extrinsic gettering refers to employing external techniques to create damage or stress in the silicon lattice such that the resulting defects in the lattice trap impurities. One example of extrinsic gettering is mechanically damaging a backside of a semiconductor wafer. For instance, abrasion, grooving or sandblasting may produce stress at the backside of the wafer. Subsequent annealing creates dislocations that tend to relieve the stresses inflicted on the wafer. These dislocations serve as gettering sites for trapping impurities.
Diffusing phosphorus into the wafer backside is another extrinsic gettering technique. This diffusion results in phosphorus vacancies or dislocations that serve as trapping sites for impurity atoms (e.g., gold). This diffusion may also form Si—P precipitates that may remove Ni impurities through interactions between Si self-interstitials and Ni atoms, nucleating NiSi2 particles in the process.
Damage may also be introduced by scanning a laser beam across the backside of the wafer. This is very similar to mechanical extrinsic gettering mechanisms, but stress on the wafer is from thermal shock caused by the laser beam. Yet another extrinsic gettering technique is ion bombardment of the wafer backside. Here, high-energy ions induce stress in the lattices of the wafer backside.
One further extrinsic gettering technique involves the deposition of a polysilicon layer on the wafer backside. The polysilicon introduces grain boundaries and lattice disorder that may act as traps for mobile impurities.
As mentioned previously, besides extrinsic gettering, there also exists intrinsic gettering. Intrinsic gettering refers to the creation of impurity trapping sites by, for example, precipitating supersaturated oxygen out of the silicon wafer. Here, wafers are created with certain levels of oxygen (e.g., 15-19 ppma). During formation of a semiconductor device, the precipitation of supersaturated oxygen causes the growth of clusters and introduces stress in the wafer. Eventually, these stresses result in dislocation loops or stacking faults that serve as trapping sites for impurities.